The present invention relates to a method for manufacturing of a packaging laminate having a thin silicone oxide coating formed on each side of a substrate plastics film, a packaging laminate manufactured by the method and a packaging container, as well as a packaging material blank, manufactured from such a packaging laminate.
Laminated packaging materials having flexibility have been used for packaging liquid food products for many years. For example, milk has been packaged in cartons made from a laminate composed of paperboard substrate with thermoplastic coatings on both surfaces. The surfaces of the carton are heatsealed together so as to form a package carton of desired shape.
Some food products, such as orange juice, packaged in such cartons, lose their nutritional values due to the permeation of oxygen through the carton walls. It has therefore been common to include an aluminum foil layer with the laminate material, in order to reduce the permeation of oxygen through the walls and to minimize the degradation of the nutrients such as vitamin C. Although aluminum foil is highly effective as a barrier material, its use in cartons may in some cases raise concern from environmental and recycling points of view and it may be deemed appropriate to replace aluminium foil by other barrier materials. Various attempts have been made to develop practical alternatives to aluminum foil. Such alternatives should have excellent oxygen, gas and aroma barrier properties, and be easily disposable after use.
Another problem in the packaging of liquid food products in cartons arises from the structure of the carton. The carton is fabricated from a carton blank or a carton web (composed of a laminate such as the ones discussed above) that is folded along one or more crease lines for formation into the desired shape. In general, portions of the blank are overlapped for sealing which may be accomplished by the application of suitable adhesive or by heat-sealing the thermoplastic layers together. The creasing of the laminate material as mentioned above imposes stresses to the laminated material. These stresses may cause leakage or, at least weaken the laminate material so that subsequent handling of the carton may lead to leakage.
New oxygen barrier materials have emerged from recent developments in plasma deposition technology for plastics films. The food and pharmaceutical packaging industries have shown tremendous interest in substrate films, usually of thermoplastic polyester, coated with a thin silicon oxide layer. These materials show excellent barrier properties as well as tolerance to the thermomechanical stress encountered during the various converting processes in the manufacture of laminated packaging materials.
U.S. Pat. No. 4,888,199 describes the process of depositing a thin film of a silicon oxide on a surface with the use of plasma under controlled conditions. The plasma is formed in a closed reaction chamber, in which the substrate is positioned. The above-mentioned substrate can be formed from metal, glass or certain plastics. The air is pumped out of the chamber until a high degree of vacuum is achieved.
For example, the organic silicon compound such as hexamethyl disiloxane is introduced into the chamber together with oxygen and helium, so the silicon molecules and oxygen molecules are deposited on the surface of the substrate. The resulting film is described as being a thin film that is very hard, scratch-resistant, optically clear and adheres well to a flexible substrate. The disclosure of the patent is hereby incorporated into this specification by reference.
An improved plasma enhanced chamical vapour deposition (PECVD) method process is described in U.S. Pat. No. 5,224,441, which is also incorporated into this specification. In the process mentioned in the patent, the substrate deposited with the silicon oxide is maintained at a temperature of about 10-35xc2x0 C., preferably 15-25xc2x0 C. and the substrate may be formed from polyethylene terephthalate (PET) or polycarbonate resin. In this specification, the thickness of the silicon oxide film when used for food packaging, is about 100 xc3x85 (Angstrom)-400 xc3x85 and the thickness of the substrate is 1.5 microns xe2x88x92250 microns.
However, during these processes, a major concern is the durability of the barrier layer in that it must not crack or delaminate (detach) from the substrate film. Tendency to cracking is controlled by the cohesion of the oxide material to itself, whereas delamination is controlled by the interfacial adhesion between the oxide layer and the substrate film. Thus there remains a need for a silicon oxide coated substrate that is resistant to cracking and delamination, for providing packages having improved gas barrier and durability properties.
In view of the deficiencies of conventional barrier laminate materials such as presented above, it is an object of this invention to provide a packaging laminate material having improved barrier and durability properties.
It is a further object of this invention to provide a laminated packaging material that is flexible, and readily capable of being formed into packages, filled and sealed, using conventional packaging machines, thus resulting in packages having improved barrier properties.
Furthermore, it is an object to provide a laminated packaging material that may be readily disposed or recycled without or with reduced detriment to the environment.
Moreover, it is further an object of the invention to provide a laminated barrier packaging material having improved durability to thermomechanical stresses encountered during various converting processes in the manufacturing of laminated packaging materials and of packaging containers.
These objects are accomplished by a packaging laminate including a substrate plastics film having a silicon oxide coating formed on each side thereof, being manufactured by a method which comprises a step for obtaining said silicone oxide coatings by means of vapour-depositing a silicon oxide coating onto each side of the substrate film by a plasma method chemical vapour deposition method (PECVD) while straining the film within a range between an upper limit of an intial plastic deformation, determined by the Young modulus of the substrate, and a lower limit of any improvement of a cohesion force in the oxide coating and an adhesion force, i.e. interfacial shear strength, between the oxide coating and the substrate film.
According to preferred and advantageous embodiments of the invention, a method and a packaging laminate as set out in claims 2-7 and claims 8-12, respectively is provided.
According to a further aspect of the invention, a packaging container manufactured from the packaging laminate of the invention is provided, as set out in claims 13-19. Furthermore, a package blank comprising the packaging laminate of the invention is provided as defined in claims 20 and 21.
The silicon oxide coatings, vapor-deposited by a plasma CVD method, are preferably carbon-containing silicone oxide coatings. The preferred carbon-containing silicone oxide has the following general formula;
SiOxCy in which x is within the range of preferably 1.5-2.2, y is within the range of 0.15-0.80, more preferably x is within the range of 1.7-2.1, y is within the range of 0.39-0.47.
The oxide coating may not be limited to these three elements due to impurities occurring throughout the manufacturing process of the laminate. The silicon oxide coating may also contain hydrogen atoms in varying amounts, depending on the precursor compounds employed.
The silicon oxide is usually formed by means of plasma discharge containing a gas mixture of oxygen, helium and organic silicon compounds (silicon precursor compounds that contain carbon). The organic silicon compounds may contain a lot of carbon atoms that participate in the plasma discharge. Some of the carbon atoms may be incorporated in the deposited layer, while the remaining carbon atoms are exhausted from the system in the form of carbon oxide (CO1 CO2), water and various gaseous hydrocarbons.
Plasma enhanced chemical vapor deposition method (PECVD) is a known technique for fabricating silicon oxide coated substrate films. The present invention improves upon that technique by straining the substrate film during deposition and controlling the quantity of oxygen in the gas mixture to create an oxide with the proper stoichiometry.
Examples of devices employed in the continuous method of manufacturing according to this invention are shown in the FIGS. 2 and 3. These examples of devices each consists of a vacuum chamber forming the process zone, a plasma generator, a pump connected to the chamber, a means of feeding the raw material mixed gas to the chamber, a drum for passing the plastics substrate film through the vacuum chamber and facilitating vapor-deposition of the oxide onto the substrate film and a roll for unwinding (feeding) the substrate film for supplying to the vacuum chamber and a rewinding roll for pulling and winding the obtained film from the vacuum chamber.
In the examples of the devices according to the invention, an electric motor for the unwinding roll and an electric motor for the rewinding roll are controlled and the strain power of the substrate film according to the invention is within the following range; The range between an upper limit showing no plastic deformation, which is determined by the Young modulus of the substrate, and a lower limit showing any improvement of a cohesion force of the oxide coating and an adhesion force between the oxide coating and the substrate.
When the device involves no drum, the straining of the substrate during plasma vapour-deposition may be carried out in a similar way.
For example, a mixture of vaporized organic silicon compound such as tetra methyl disiloxane (TMDSO) or hexa methyl disiloxane (HMDSO) and inert gas (e.g. helium) and oxygen gas is fed into the vacuum chamber. Preferably, the organic silicon compound is tetra methyl disiloxane (TMDSO). When the plasma is ignited, the vaporized silicon compound reacts with the oxygen to form a silicon oxide compound, which is bonded to the cool substrate film in the vacuum chamber.
By regulating the quantity of oxygen in the gas mixture that is fed into the vacuum chamber, it is possible to control the chemical reaction within the vacuum chamber so that the thus-formed silicon oxide can have a formula SiOxCy, in which x is within the range of 1.5-2.2 and y is within the range of 0.15-0.80, and more preferably x is 1.7-2.1 and y is within the range of 0.39-0.47. It has been proved that the carbon-containing silicon oxide formed within this range has optimal oxygen gas and aroma barrier properties.
The result of measurements of the average atom concentrations, when analyzing the carbon-containing silicon oxide coating obtained by the method of manufacturing according to this invention by the ESCA method, has shown 30.1xc2x15.0% of silicon, 57.3xc2x15.0% of oxygen, 12.6xc2x15.0% of carbon. From a stoichiometric perspective, the carbon containing silicon oxide has an average stoichiometry of SiO1.90C0.419.
The carbon-containing silicon oxide coating is obtained by vapor-deposition onto the substrate film by means of a plasma CVD method while straining the substrate film within a range between the upper limit showing no plastic deformation, determined by the Young modulus of the substrate, and the lower limit showing any improvement of a cohesion force of the oxide coating and the adhesion force between the oxide coating and the substrate.
The silicon oxide compound is directly formed on the surface of the substrate. The compactness of the thus-formed silicon oxide layer on the substrate or the core layer becomes sufficiently high from a barrier perspective once the strained substrate film is released or no longer under the tensioning force. As a result of this process, the silicon oxide layer can be made very thin without any loss of the desired barrier properties.
The preferable substrate is made of a flexible thermoplastic material, such as polyethylene, polypropylene or polyethylene terephthalate (PET), preferably of PET.
The silicon oxide layer formed by the PECVD method according to this invention is able to withstand substantial elongation without rupture. This characteristic is especially important for use of the laminate material in the packaging of liquid food products, since the typical packaging laminate material has crease lines formed in the surface of the laminate to facilitate the bending and folding of the material into a package. The ability of the silicon oxide layer to be deformed without rupture substantially decreases the possibility of leakage along the crease lines.
By employing double layers of such carbon-containing silicon oxide barrier coatings produced by plasma chemical vapor deposition while stretching the substrate film within the range as set out above, in a packaging laminate, the risk of cracks appearing in the barrier layers and delamination from the substrate layer due to folding and bending of the material may be reduced even further.
Normally, the substrate film will be coated on each side in a separate plasma CVD coating step, i.e. the plasma coating process is a batchwise process.
A particular advantage with the method according to the present invention is, that the firstly applied silicon oxide coating on the first side of the substrate film will be flexible and durable and thus not crack or detach from the substrate film during the operation of coating the second silicon oxide coating on the second side of the substrate film. Consequently, a flexible film having an intact, flexible and durable silicon oxide coating on each side will be obtained.